Process for obtaining a layer of single-crystal germanium or silicon on a substrate of single-crystal silicon or germanium, respectively

ABSTRACT

The process consists in depositing, by chemical vapor deposition using a mixture of silicon and germanium precursor gases, a single-crystal layer of silicon or germanium on a germanium or silicon substrate by decreasing or increasing the temperature in the range 800-450° C. and at the same time by increasing the Si/Ge or Ge/Si weight ratio from 0 to 100% in the precursor gas mixture, respectively.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates in a general way to a process forobtaining a layer of single-crystal germanium on a substrate ofsingle-crystal silicon or, conversely, a layer of single-crystal siliconon a substrate of single-crystal germanium.

2. Description of the Related Art

Silicon (Si) is the basic compound of microelectronics. It is currentlyavailable on the market in the form of wafers 200 mm in diameter. Theperformance limits of integrated circuits are in fact therefore thoseassociated with the intrinsic properties of silicon. Among theseproperties, mention may be made of the electron mobility.

Germanium (Ge), which belongs to column IV of the Periodic Table ofElements, is a semiconductor. It is potentially more beneficial than Sisince (i) it has a higher electron mobility, (ii) it absorbs well in theinfrared range and (iii) its lattice parameter is greater than that ofSi, thereby allowing heteroepitaxial structures using the semiconductormaterials of columns III-V of the Periodic Table.

Unfortunately, germanium does not have a stable oxide and there are nohigh-diameter germanium wafers on the market, except at prohibitiveprices.

Si_(1-x) Ge_(x) alloys have already been grown on substrates ofsingle-crystal Si. The alloys obtained only rarely have germaniumcontents exceeding 50% in the alloy.

Moreover, when SiGe alloys are grown on single-crystal Si, the growth ofthe SiGe alloy is initially single-crystal growth. The greater thethickness of the layer and the higher its germanium content, the morethe layer becomes "strained". Above a certain thickness, the "strain"becomes too high and the layer relaxes, emitting dislocations. Thesedislocations have a deleterious effect on the future circuits which willbe constructed on this layer and the relaxation of the layers causescertain advantages of the strained band structure (offsetting of theconduction and valence bands depending on the strain states: Si/SiGe orSiGe/Si) to be lost. Corresponding to each composition and to eachproduction temperature there is therefore a maximum thickness orstrained layer.

In some applications, the concept of "relaxed substrates" has beendeveloped, that is to say Si_(1-x) Ge_(x) layers are grown on silicon soas to exceed the critical thickness for a given composition, but byadjusting the deposition parameters for the layers so that thedislocations emitted do not propagate vertically but are bent over so asto propagate in the plane of the layer in order subsequently toevaporate at the edges of the wafer. Growth therefore takes place fromincreasingly germanium-rich layers, it being possible for the germaniumgradient to change stepwise or in a continuous fashion.

However, the layers deposited by this "relaxed substrate" process eitherhave a relatively low (<50%) degree of germanium enrichment or have anunacceptable density of emergent dislocations for applications inmicroelectronics.

Thus, the article entitled "Stepwise equilibrated graded Ge_(x) Si_(1-x)buffer with very low threading dislocation density on Si (001), by G.Kissinger, T. Morgenstern, G. Morgenstern and H. Richter, Appl.Phy.Lett.66(16), Apr. 17, 1995", describes a process in which the sequence of thefollowing layers is deposited on a substrate:

250 nm Ge₀.05 Si₀.95 +100 nm Ge₀.1 Si₀.9 +100 nm Ge₀.15 Si₀.85 +150 nmGe₀.2 Si₀.8.

After each layer has been deposited, it undergoes in situ annealing inhydrogen at 1095 or 1050° C. By way of comparison, similar sequences oflayers have been deposited, but without annealing.

A 300 nm layer of Ge_(x) Si_(1-x) of the same composition as the upperbuffer layer is also deposited on the latter.

The specimens which did not undergo intermediate annealing have anemergent-dislocation density of 10⁶ cm⁻², whereas the specimen whichunderwent annealing has an emergent-dislocation density of 10³ -10⁴cm⁻².

The article entitled "Line, point and surface defect morphology ofgraded, relaxed GeSi alloys on Si substrates", by E. A. Fitzgerald andS. B. Samavedam, Thin Solid Films, 294, 1997, 3-10, describes themanufacture of relaxed substrates comprising up to 100% germanium.However, the process employed takes a long time (more than about 4 hoursper wafer) and is consequently unattractive from an industrialstandpoint. Moreover, this process is not reversible, that is to say itdoes not allow pure silicon to be deposited on a germanium substrate.

Furthermore, during the fabrication of such relaxed substrates, asurface roughness is observed which increases depending on thedeposition conditions and which may have negative defects--since theyare cumulative--that is to say an onset of roughness can but increaseduring definition.

SUMMARY OF THE INVENTION

In one embodiment a process for obtaining, on a substrate ofsingle-crystal silicon, a Si_(1-x) Ge_(x) layer which has a highgermanium content and which may be pure germanium, having a lowemergent-dislocation density, and vice versa is described.

In one embodiment a process for obtaining a Si_(1-x) Ge_(x) layer havinga high germanium content and a very low surface roughness is described.

In one embodiment a process as defined above which may be implemented inan industrial reactor, for example an industrial single-wafer reactor isdescribed.

The process for obtaining a layer of single-crystal germanium or ofsingle-crystal silicon on a substrate of single-crystal silicon or ofsingle-crystal germanium, respectively, includes the chemical vapourdeposition of a layer of single-crystal silicon or germanium using amixture of germanium and silicon precursor gases, the said process beingcharacterized in that:

a) in the case of deposition of the layer of single-crystal germanium,the deposition temperature is gradually reduced in the range of 800° C.to 450° C., preferably 650 to 500° C., while at the same time graduallyincreasing the Ge/Si weight ratio in the precursor gas mixture from 0 to100%; and

b) in the case of deposition of the layer of single-crystal silicon, thedeposition temperature is gradually increased in the range of 450 to800° C., preferably 500 to 650° C., while at the same time graduallyincreasing the Si/Ge weight ratio in the precursor gas mixture from 0 to100%.

Any Si and Ge precursor gas, such as SiH₄, Si₂ H₆, SiH₂ Cl₂, SiHCl₃,SiCl₄, Si(CH₃)₄ and GeH₄, may be used in the process.

The preferred precursors are SiH₄ and GeH₄.

As is well known, the precursor gases are preferably diluted in acarrier gas such as hydrogen. The dilution factors may vary from 10 to1000.

The chemical vapour deposition preferably takes place at low pressure,typically 8 kPa, but may also be carried out at atmospheric pressure byadapting the gas phases.

It has been determined that a pressure of about 8 kPa (60 torr) gave thebest compromise between a high growth rate of the layers and depositioncontrol.

Again preferably, the surface of the substrate is subjected to apreparation step prior to deposition.

This preparation step may conventionally be a surface cleaning step, forexample any process in the liquid or gas phase which cleans the siliconsurface of the metallic and organic residues, such as the conventionalsolutions SC1 (NH₄ OH+H₂ O₂) and SC₂ (HCl+H₂ O₂) or else H₂ SO₄ +H₂ O₂.In all cases, the cleaning is completed by a phase of treatment using adilute HF aqueous solution followed by rinsing in water.

Preferably, the process is carried out in stages of defined durationduring which the temperature and the gas fluxes are modified linearly asa function of time. In other words, in the case, for example, ofdeposition of a layer of pure germanium on a silicon substrate, thetemperature is lowered from the maximum deposition temperature to theminimum temperature in stages of defined duration during which thetemperature is reduced linearly from a first value to a second value.During this same time interval, the Ge/Si weight ratio in the precursorgas mixture is increased linearly, for example by varying the fluxes ofthe precursor gases. The last stage is that for which the minimumdeposition temperature has been reached and in which the Ge/Si weightratio of the precursor gas mixture is 100/0. The duration of this laststage depends on the desired thickness of the layer of pure germanium.

The number and duration of the stages may be determined depending on thetotal duration of the deposition process and on the optimization of thequality of the material deposited. In general, a total deposition timeof approximately one hour is chosen, which corresponds to anindustrially acceptable compromise between the quality of the materialdeposited (minimum surface roughness for a given total thickness) andtotal deposition time.

It is possible, if desired, to insert, between each variable-temperatureand variable-flux stage, fixed-temperature but variable-flux depositionstages, or vice versa.

In order to grow pure Si on pure Ge, all that is required is to carryout the process as before but by reversing the directions of thetemperature and flux variations.

It is therefore possible, to produce successive stacks of layers of puregermanium and of pure silicon on a substrate of single-crystal germaniumor of single-crystal silicon.

Preferably, the process is carried out in a single-wafer reactor whichallows greater controllability of the parameters (more rapid change ofthe gas compositions and of the temperature). However, any othersuitable device such as, for example, a furnace may be used.

In another embodiment, the above deposition steps (a) and (b) arecarried out alternately in order to obtain a multilayer product havingalternate layers of single-crystal silicon and single-crystal germanium.

In another embodiment multilayer products may be formed which include,for example, stacks of the following structures:

Si(single crystal)/Si_(1-x) Ge_(x) (x varying from 0 to 1)/Ge (singlecrystal); Si(single crystal)/Si_(1-x) Ge_(x) (x varying from 0 to1)/Si_(1-y) Ge_(y) (y varying from 1 to 0)/Si(single crystal); Si(singlecrystal)/Si_(1-x) Ge_(x) (x varying from 0 to 1)/Ge(singlecrystal)/Si_(1-y) Ge_(y) (y varying from 1 to 0)/Si(single crystal);Ge(single crystal)/Si_(1-x) Ge_(x) (x varying from 1 to 0)/Si(singlecrystal); Ge(single crystal)/Si_(1-x) Ge_(x) (x varying from 1 to0)/Si_(1-y) Ge_(y) (y varying from 0 to 1)/Ge(single crystal); Ge(singlecrystal)/Si_(1-x) Ge_(x) (x varying from 1 to 0)/Si(singlecrystal)/Si_(1-y) Ge_(y) (y varying from 0 to 1)/Ge(single crystal); andcombinations of these stacks.

The multilayer products generally have an emergent-dislocation density≦10³ /cm².

Although the process described above limits the appearance of a roughsurface, it is again desirable to reduce the surface roughness of theSi_(1-x) Ge_(x) deposited.

Thus, the wafer specimens obtained by the process described above mayhave two surface roughnesses, namely a low roughness in the form of"hachured fabrics" of low amplitude (<60 nm) and a high roughness (>100nm peak-to-valley) of longer wavelength.

In order to eliminate this roughness, a chemical-mechanical polishingstep may be provided which eliminates either only the high roughness orall of the roughness. This chemical-mechanical polishing step may beimplemented on Si_(1-x) Ge_(x) layers for all germanium concentrations.Thus, the chemical-mechanical polishing may be carried out on a pure Geor Si layer or on any intermediate layer before the pure Ge or Si layeris deposited. Furthermore, it is preferred to carry out thischemical-mechanical polishing stage on an Si_(1-x) Ge_(x) layer forwhich x<1 or x>0, thereby making it possible to deposit, on the Si_(1-x)Ge_(x) layer thus polished, by means of the process described above,relaxed layers of Si_(1-x) Ge_(x) with increasing Ge or Siconcentrations, starting from a deposited material having the same Geand Si concentration as the polished layer, until a layer of pure Ge orSi is obtained. The final layer thus obtained is practically free ofroughness.

Any type of chemical-mechanical polishing conventionally used in silicontechnology may be used.

The principle of chemical-mechanical polishing is known and conventionaland includes rubbing the wafer to be polished on a tissue imbibed withabrasive, applying pressure and moving this wafer with respect to thetissue. For further details, reference may be made to the Patents. Theconjugate mechanical and chemical effects cause molecules of thepolished materials to be preferentially removed from the regions inrelief and planarize the material to be polished.

The polishing is controlled either in situ by control of polishing data,such as the current for the motors, or ex situ in a qualitative mannerby optical or microscopic observations and/or in a quantitative mannerby a technique of atomic force microscopy [measurement of the average(rms) or peak-to-valley roughness].

After polishing, encrusted mechanical residues may remain on thesurface, which will be removed by mechanical brushing and rinsing.

After this cleaning, the polishing may leave a disturbed surface regionand a treatment to regenerate the surface may be necessary. Thistreatment, which will be of the etching type, must nevertheless becarried out without causing the entire active layer to disappear.Several methods are possible.

It is possible either (i) to etch, by dry or wet etching, the layer or(ii) to oxidize the surface and then dissolve the oxide. Both thesemeans will use the extreme sensitivity of Ge to oxygen (gaseous oxygenor ozone, or ozone dissolved in water, or plasma, etc.), the oxides ofGe being volatile or unstable.

After these treatments, epitaxial growth on the surface may be resumed,particularly using the process described above. In this preferred case,the desired surface finish (counting of defects) and therefore a"guaranteed" layer, the thickness of which may be adjusted depending onthe envisaged application, is therefore immediately obtained. Inaddition, impurities are trapped by the subjacent dislocation network.

As indicated previously, the technique described above can be appliedfor any Ge concentration in the Si_(1-x) Ge_(x) layer and gives verygood results in terms of planarity and residual surface defects.

Nevertheless, for Ge concentrations typically above 70%, "holes" mayappear after polishing, these corresponding to defects which weretransferred into the upper layer. The density of these holes is about10⁴ to 10⁵ df/cm². These holes are obviously not polished. These defectsare probably stacking faults and/or defects associated with a linking,between two orthogonal guide planes (<110> directions), of thedislocations emitted in order to relax the growing structure. Two typesof "holes" may exist:

(i) holes which have a depth not exceeding the upperconstant-composition layer. In order to obviate these holes, asufficient thickness of the desired final composition of the upper layer(typically pure Ge) must be deposited and polished until the "holes"disappear;

(ii) "deeper" holes in the form of an upside-down pyramid [on asubstrate of (100) single-crystal Si]; the apex of the upside-downpyramid lies in the graded-composition layers, usually those with a Geconcentration above 55%. These holes, with sharp rectangular edges, arerounded in various ways by the polishing, but to the detriment of agreater region of extension. In order to limit these holes, onetechnique includes introducing, into the growth stages above a Geconcentration of 55%, steps in which the composition is constant(typically 300 nm every 10%, thereby increasing the process time perwafer, which time nevertheless remains acceptable). This has the effectof reducing the density of these defects;

(iii) another possibility for limiting the extension of the "deeper"holes includes in fabricating a substrate with concentrations of up toabout 70% and polishing, cleaning and resuming the growth by increasingthe Ge concentration of the layers up to that desired. This may becarried out as many times as necessary. Thus, lower defect densities anda lower extension of these defects are obtained.

This variant of the process has several advantages;

Polishing machines and solutions conventionally used in silicontechnology may be used for chemical-mechanical polishing;

Only very little material need be polished, this furthermore appearingas peak-to-valley material (in general about 200 nm). This leaves agreat deal of freedom and allows the use of CMP solutions identical tothose for silicon;

The disappearance of the "work-hardened" region after polishing is easyto achieve since compounds of Ge with oxygen are very unstable, i.e.they dissolve in any process containing oxygen (thermal oxidation orplasma-assisted oxidation, dissolution in ozonated water or in chemicalsolutions which selectively etched Ge, etc.)

It involves "light" polishing, that is to say a process with a highdegree of freedom in the choice of thickness and of uniformity, if careis taken to initially produce a relatively thick layer to be polished(typically more than twice the peak-to-peak roughness to be removed);

Repeating the epitaxial growth of a layer having the same composition asthat which has just been polished will "guarantee" the surface and makeit possible to continue with other variations (other relaxed or strainedlayers above it), that is to say the starting surface is one without anyroughness;

This technique may be extended to the reverse situation, that is to sayto the formation of Si layers on Ge.

BRIEF DESCRIPTION OF THE DRAWINGS

The rest of the description refers to the appended figures which show,respectively;

FIG. 1--a graph of the deposition temperatures as a function of time(curve A) and a graph of the ratio of the precursor gas (GeH₄ /SiH₄)flow rates as a function of time (curve B);

FIG. 2--graphs of the SiH₄ precursor gas flow rate (curve C) and GeH₄precursor gas flow rate (curve D) as a function of time, together with agraph of the ratio of the flow rates of these precursor gases as afunction of time (curve B); and

FIG. 3--a photomicrograph (10,000×) of a substrate coated with a stackof Si_(1-x) Ge (x varying from 0 to 1)/GeSi_(1-x) Ge_(x) (varying from 1to 0)/Si layers.

DETAILED DESCRIPTION OF THE INVENTION

The process will now be described, with reference to FIGS. 1 and 2, inthe case of the deposition of germanium on a substrate of single-crystalsilicon.

The precursor gases used for the chemical vapour deposition are GeH₄ andSiH₄. The carrier gas is hydrogen (H₂) at a flow rate of 60 l/minute.The total pressure in the reactor is 8 kPa (60 torr).

As shown in FIG. 2, the flow rates of SiH₄ and of GeH₄ diluted to 10% inH₂ vary between 0 and 200 cm³ /minute and between 0 and 500 cm³ /minuterespectively.

Referring to FIG. 1, the temperature of the chemical vapour depositionis varied from 650° C. to 500° C. in stages while at the same timevarying the Ge/Si weight ratio in the precursor gas mixture, for exampleby varying the SiH₄ and GeH₄ flow rates, as shown in FIG. 2 by curves Cand D.

Part a of curve A in FIG. 1 corresponds to the deposition at 650° C. ofan epitaxially grown layer of pure silicon (buffer layer) serving solelyto ensure that there is a good start for the subsequent epitaxialgrowths. This buffer layer may be omitted, depending on the quality ofthe cleaning of the Si substrate or of the Si wafers supplied by themanufacturer.

The part b corresponds to the deposition at 500° C. of a layer of puregermanium. The thickness of this layer will quite obviously depend onthe duration of this deposition phase and is a function of thesubsequent use envisaged.

In the implementation shown in FIGS. 1 and 2, the temperature is variedin stages and, at the same time, the Ge/Si weight ratio in the precursorgas mixture may be varied in stages.

Thus, in order to go from a temperature of 650° C. to 620° C., the twotemperatures and the time for going from 650° C. to 620° C. are fixed.There is linear interpolation between the two temperatures.

The same procedure is employed for varying the respective flow rates ofthe precursor gases. Thus, as shown in FIG. 2, during the same timeinterval in which the temperature is lowered from 650° C. to 620° C.,the GeH₄ flow rate is increased from 0 to 70 cm³ /minute while stillmaintaining the SiH₄ flow rate at 200 cm³ /minute, thus increasing thegermanium content in the GeSi alloy layer deposited.

This procedure may be repeated in temperature stages while increasingthe GeH₄ flow in stages until reaching 500 cm³ /minute, for a chosentemperature (for example, a temperature of 560° C.). Above thistemperature, the flow of GeH₄ is maintained at 500 cm³ /minute and theflow of SiH₄ is lowered in stages from 200 cm³ /minute to 0, while atthe same time the temperature is lowered in stages from 560 to 500° C.

Preferably, the temperature reduction curve must lie below the straightline joining the extreme temperatures, that is to say it must be aconcave curve. The greater the concavity, the better the quality of thelayers deposited (minimum roughness for a given total thickness).However, the greater this concavity, the longer the process. In general,a concavity will be chosen, and consequently a number of stages, suchthat the duration of the deposition process is acceptable from anindustrial standpoint, for example approximately one hour.

The process, illustrated in FIGS. 1 and 2, was implemented in asingle-wafer (200 mm wafer) reactor with a leakage rate of less than1.33 Pa/minute (10 mτ/minute) in order to avoid H₂ O and O₂contamination deleterious for germanium.

FIG. 3 is a photomicrograph of a preferred multilayer product accordingto the invention.

The roughness seen in the photomicrograph (whether in the plane ofsection or on the surface) merely results from chemical decoration ofthe specimen, the purpose of this decoration simply being to increasethe visual contrast between the various layers of the structure.

This multilayer product comprises the following structure:

Si(single-crystal substrate)/Si_(1-x) Ge_(x) (x varying from 0 to1)/Ge(single-crystal)/Si_(1-y) Ge_(y) (y varying from 1 to0)/Si(single-crystal).

There is no network of emergent dislocations in this multilayer product.

The chemical-mechanical polishing of the multilayer wafers obtained bythe above process, having an upper layer of pure Ge with a thickness ofabout 400 nm, was carried out on an industrial machine of the Presibrand, of the E550 type, with a Rodel tissue, reference IC400 (aso-called "hard" tissue) and a Clariant abrasive, reference Klebosol30N50PHN (a standard abrasive in silicon technology). Typical processparameters used for polishing the multilayer wafers are the following:pressure of 0.3 DaN/cm², plate-and-head speed of 0.8 m/sec andtemperature of 15° C. After polishing, a rinsing step is carried out ata pressure of 0.15 DaN/cm² and at speeds of 1.4 m/sec, the abrasivebeing replaced by deionized water. After polishing, the wafers arebrushed (in deionized water and NH₄ OH) and dried in a machine of theOntrack brand, SS200 type, in a conventional manner.

Of course, the various parameters mentioned above may be modified to alarge extent depending on the polished layer and on the result desired.

The SiGe etching rate adopted is ≦80 nm/min with a non-uniformity ofless than 3% on wafers 200 mm in diameter.

The 0.2 μm particle content (excluding deep holes) is less than 0.8defects/cm² and no microscratching was observed.

The planarization of the peaks after chemical-mechanical polishingresults in a roughness of about 1 nm (root mean square value) afterpolishing for 5 minutes, having started with a Ge layer havingpeak-to-valley variations of 200 nm.

Further modifications and alternative embodiments of various aspects ofthe invention will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as the presently preferred embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

What is claimed is:
 1. Process for obtaining a layer of single-crystalgermanium or single-crystal silicon on a substrate of single-crystalsilicon or of single-crystal germanium, respectively,comprising:chemical vapour depositing the layer of single-crystalgermanium or single-crystal silicon using a mixture of silicon andgermanium precursor gases, wherein:a) in the case of deposition of thelayer of single-crystal germanium, the deposition temperature isgradually decreased from about 800° C. to about 450° C., while at thesame time gradually increasing the Ge/Si weight ratio in the precursorgas mixture from 0 to 100%; and b) in the case of deposition of thelayer of single-crystal silicon, the deposition temperature is graduallyincreased from about 450° C. to about 850° C., while at the same timegradually increasing the Si/Ge weight ratio in the precursor gas mixturefrom 0 to 100%.
 2. Process according to claim 1, wherein the temperatureis increased or decreased linearly in stages of the desired duration andthe Ge/Si or Si/Ge weight ratio is increased linearly during thesestages.
 3. Process according to claim 1, wherein the Ge/Si and Si/Geweight ratios are increased by modifying the flow rates of the precursorgases.
 4. Process according to claim 1, wherein the precursor gases areSiH₄ and GeH₄.
 5. Process according to claim 1, wherein the chemicalvapour deposition is implemented in a single-wafer reactor.
 6. Processaccording to claim 1, wherein the deposition steps (a) and (b) arecarried out alternately in order to obtain a multilayer product havingalternate layers of single-crystal silicon and single-crystal germanium.7. Process according to claim 1, further comprising chemical-mechanicalpolishing of the layer of single-crystal germanium or of single-crystalsilicon.
 8. Process according claim 1, further comprising one or moresteps of chemical-mechanical polishing of the coated surface of thesubstrate before deposition of the pure Ge or pure Si.
 9. Processaccording to claim 1, wherein in the case of deposition of the layer ofsingle-crystal germanium, the deposition temperature is graduallydecreased from about 650° C. to about 500° C.
 10. Process according toclaim 1, wherein in the case of deposition of the single-crystalsilicon, the deposition temperature is gradually increased from about500° C. to about 650° C.
 11. Process for obtaining a layer ofsingle-crystal germanium on a substrate of single-crystal silicon,comprising:chemical vapour depositing the layer of single-crystalgermanium upon the substrate of single crystal silicon using a mixtureof silicon and germanium precursor gases, wherein the depositiontemperature is gradually decreased while at the same time graduallyincreasing the Ge/Si weight ratio in the precursor gas mixture from 0 to100%.
 12. The process of claim 11, wherein the deposition temperature isdecreased from about 800° C. to about 450° C.
 13. The process of claim11, wherein the Ge/Si weight ratio in the precursor gas mixture isincreased by modifying the flow rates of the precursor gases.
 14. Theprocess of claim 11, wherein the precursor gases are SiH₄ and GeH₄. 15.A process for obtaining a layer of single-crystal silicon on a substrateof single-crystal germanium, comprising:chemical vapour depositing thelayer of single-crystal silicon upon the substrate of single-crystalgermanium using a mixture of silicon and germanium precursor gases,wherein the deposition temperature is gradually increased while at thesame time gradually increasing the Si/Ge weight ratio in the precursorgas mixture from 0 to 100%.
 16. The process of claim 15, wherein thedeposition temperature is gradually increased from about 450° C. toabout 800° C.
 17. The process of claim 15, wherein the Si/Ge weightratio in the precursor gas mixture is increased by modifying the flowrates of the precursor gases.
 18. The process of claim 15, wherein theprecursor gases are SiH₄ and GeH₄.